... HST . ... HST Overview . ... Retrieving HST Data . ... The second is the internal geometric accuracy of an FOC image itself, including the correctness of the distortion model, the plate scale, and the image rotation. ... The accuracy with which HST places a target in the FOC field of view depends in a complex way on the target coordinate uncertainty, the positions of the guide stars in the FGS fields of view, and the alignment of the FOC imaging aperture with respect to the FGS reference frame. ...